1. Field of the Invention
The field of the present invention relates to large-area OLEDs (organic light-emitting diodes) with homogeneous light emission and to processes for the production thereof.
2. Background
OLEDs have undergone rapid development in recent years. They are increasingly replacing the liquid-crystal displays (LCDs) currently employed. OLEDs have a number of advantages over LCDs: they are distinguished on the one hand by a simpler structure and by lower energy consumption. In addition, they have a smaller viewing-angle dependence.
In addition, the illumination sector is apparent as a further area of application of OLEDs. They are replacing both conventional illuminants and also LEDs here.
Compared with conventional illuminants, such as, for example, incandescent bulbs and halogen tubes, OLEDs are distinguished by a much thinner structure. In addition, OLEDs are two-dimensional illuminants, in contrast to the punctiform, conventional illuminants. This makes it possible to use OLEDs in areas of application in which conventional illuminants cannot be employed.
Compared with LEDs, OLEDs have virtually Lambert emission characteristics, i.e. there is no need to use, as in LEDs, complex optics which convert a point emitter into a two-dimensional emitter.
In addition, the electroluminescent spectrum of an OLED is very broad compared with LEDs. This enables a wide variety of colour variations of an emitter through the use of coloured filters (“colour branding”), giving rise to high potential for OLEDs in the illumination sector.
Furthermore, OLEDs have the advantage that the shape and dimension of their emitting area can very easily be adapted for specific illumination applications by structuring of the anode and/or cathode.
In spite of the above-described advantages of OLEDs, only OLEDs having very small emitting areas are currently employed, both in the display sector and also in the illumination sector. Thus, for example, OLEDs have been used for some time in displays of mobile telephones and MP3 players.
Although OLEDs should in principle be suitable for large areas owing to their Lambert emission characteristics, without additional optics, their use in illumination applications is currently restricted to small areas. It is currently not possible to provide individual OLEDs with a large emitting area which have satisfactory homogeneity of light emission over the entire emitting area. This is attributable to the fact that the size of the emitting area is restricted, inter alia, by the use of ITO (“indium tin oxide”) as the preferred transparent anode material, since ITO has a relatively high surface resistance, which prevents homogeneous light emission over a large area.
Consequently, large areas currently cannot be illuminated homogeneously by a single large OLED. For this purpose, a plurality of OLEDs must at present be correspondingly arranged and provided with contacts.